Oils obtained from plant seeds are important sources of fatty acids for human consumption and for use as chemical feedstocks. These fatty acids include essential fatty acids, saturated fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids. In plant seed oils, fatty acids are stored predominantly as triacylglycerols (TAGs). TAGs represent the most efficient form of stored energy in eukaryotic cells.
TAG biosynthesis occurs mainly in the endoplasmic reticulum (ER) of the cell using acyl-CoA and sn-glycerol-3-phosphate as primary substrates. Biosynthesis of TAG is effected through a biochemical process generally known as the Kennedy pathway (Kennedy, 1961) which involves the sequential transfer of fatty acids from acyl-CoAs to the glycerol backbone (acyl-CoA-dependent acylation). The pathway starts with the acylation of sn-glycerol-3-phosphate to form lysophosphatidic acid through the action of sn-glycerol-3-phosphate acyltransferase. The second acylation is catalyzed by lysophosphatidic acid acyltransferase, leading to the formation of phosphatidic acid which is dephosphorylated by phosphatidate phosphatase1 to form sn-1,2-diacylglycerol. The final acylation is catalyzed by diacylglycerol acyltransferase (DGAT; EC 2.3.1.20) to form TAG. The DGAT enzyme catalyzes the transference of the acyl group from acyl-coenzymeA (acyl-CoA) donor to a sn-1,2-diacylglycerol, producing CoA and TAG. Previous research results suggest that the level of DGAT activity may have a substantial effect in the flow of carbon into seed oil (Ichihara and Noda, 1988; Perry and Harwood, 1993; Stobart et al., 1986; Settlage et al., 1998).
Two types of DGAT (DGAT1 and DGAT2) have been identified in animals and plants (Cases et al., 2001; Hobbs et al., 1999; Lardizabal et al., 2001; Kroon et al., 2006; Shockey et al., 2006). DGAT1 has been most studied and displays broad substrate specificity. DGAT1 null mutants in plants and animals have been shown to have substantially reduced levels of TAG (Routaboul et al., 1999; Smith et al., 2000). Furthermore, over-expression of DGAT1 in seeds of Arabidopsis thaliana results in increased seed weight and oil content (Jako et al., 2001). These results suggest that DGAT1 is the predominant type, although some studies indicate that DGAT2 might be more important for TAG biosynthesis in plants like castor bean (Kroon et al., 2006).
Flax is an oilseed that substantially accumulates α-linolenic acid (α-18:3) which is an omega-3 fatty acid. Other omega-3 fatty acids include eicosapentaenoic acid (EPA) and docosahexaneoic acid (DHA) which produce beneficial health effects in humans (Simopoulos, 2002). Flaxseed oil displays chemical attributes which are advantageous for industrial applications including, for example, the production of linoleum, preservation of concrete and as an ingredient in paints and varnishes. The enzymatic activity of DGAT has been studied in isolated ER of flax developing seeds (Sorensen et al., 2005). DGAT is able to incorporate polyunsaturated fatty acids (C18:3 n-3) at higher rates compared to monounsaturated (C18:1) fatty acids. In addition, flax microsomes incorporate EPA and DHA into TAGs (Sorensen et al., 2005), highlighting the usefulness of TAG biosynthetic enzymes such as DGAT as genetic tools for engineering vegetable oils. Over-expression of DGAT in oilseed plants could potentially increase TAG production or enhance seed oil content in plants. However, since numerous enzymatic activities occur within microsomes, it is difficult to evaluate the effect of DGAT in flax using a microsome-based system. Genetically modified organisms have not achieved widespread public acceptance; however, use of native flax DGAT genes for improving the oil content through biotechnology may more readily meet stringent controls.